Layer thermistors and method for their manufacture

ABSTRACT

A METHOD OF MANUFACTURING A LAYER THERMISTOR WHICH PERMITS PRECISE DETERMINATION OF ITS OHMIC VALUE COMPRISING THE STEPS OF COATING A REFRACTORY SUBSTRATE WITH A THIN SEMI-CONDUCTIVE LAYER HAVING A HIGH TEMPERATURE COEFFICIENT, BAKING SAID LAYER, MEASURING THE OHMIC VALUE OF SAID LAYER, AND ADJUSTING SAID OHMIC VALUE TO A PREDETERMINED VALUE BY CUTTING A THREAD INTO SAID LAYER WHILE MAINTAINING THE TEMPERATURE OF SAID LAYER SUBSTANTIALLY CONSTANT.

United States Patent 3,555,671 LAYER THERMISTORS AND METHOD FOR THEIR MANUFACTURE John Aizicovici, Boulogne-Billancourt, France, assiguor to Societe le Carbone Lorraine, Paris, France, a French body corporate No Drawing. Filed June 19, 1967, Ser. No. 647,188 Claims priority, applicfiatig ll France, July 1, 1966, 7 3

Int. Cl. 1301c 7/04 US. Cl. 29612 1 Claim ABSTRACT OF THE DISCLOSURE Layer thermistors are manufactured by coating a refractory substrate with a semiconductive layer and forming a thread thereon.

The present invention relates to layer thermistors. More particularly it relates to precision layer thermistors and to a method for their manufacture.

The spread of ohmic values of thermistors which are conductive in the mass after their baking is well known and largely governs the profitability of this manufacture. Many semiconductors of similar structure have electrical characteristics usually found in commerce over very wide limits. Varistors may be cited for which it is accepted that constant voltage current can range between two limits, one of which is at least double the other. Also, transistors have gains whose maxi-mini ratio is of the order of three.

Thermistors on the other hand are sold with tolerances derived from fixed resistors, most often There is a great demand for narrower tolerances, but it is difficult to meet this demand by simple sorting of the baked products, in a profitable manner and at non-prohibitive cost.

It is an object of the present invention to enable the manufacture of thermistors with narrower tolerances under industrial conditions.

According to the present invention there is provided a method of manufacturing layer thermistors, permitting the ohmic value to be determined with a high degree of precision comprising coating a refractory substrate with a semiconductive layer, baking said layer and forming a thread thereon. The substrate may be of various geometric shapes e.g. a rod, disc, rectangle or sphere made of refractory material. After baking the coated rod, this layer is measured in ohmic value and the rod is threaded with an adequate pitch for the purpose of obtaining the desired ohmic value which is higher than the original ohmic value. I

Thermistors are made in most cases from mixtures of transition metal oxides. These fine powders are generally compressed or spun. In the present case, a paste is made from the fine powder with a liquid and a binder, the viscosity being adapted to the coating operation on the material serving as substrate. This paste is deposited on the rod by projection by means of a spray gun, by immersion, or with the aid of a brush in contact with a rod rotating about its axis. Other means can be imagined and are included in the present invention on the condition that they produce layer thermistors.

3,555,671 Patented Jan. 19, 1971 Baking is effected according to the basic materials given as if the thermistors were conductive in the mass. The atmosphere can be reducing or oxidizing according to the nature of the semiconductibillity which is intended to be given to the materials. The ohmic value of the layer can be regulated either by the thickness of the deposit or by judicious variation of the proportions of the ingredients. The thickness of the layer may be between 0.05 and 0.30 mm. After baking, the threading must take into account the ohmic value to be achieved and the heating of the member, either by correcting or by eliminating said heating with the aid of an adequate grinding wheel and of a cooling in air at constant temperature. With an adequate electronic apparatus, thermistors can be manufactured under these conditions having a tolerance of i5, i2, :1, 20.5% and less.

In order that the invention may be more clearly understood the following embodiment is given by way of indicative but non-limiting example.

EXAMPLE A mixture containing equal weights of titanium oxide and, chromium oxide is used. This mixture is converted into liquid paste with the aid of a solution of distilled water containing 2% of carboxymethylcellulose. This paste is deposited on a rod made of ceramic material with a diameter of 4 mm. and length of 25 mm., rotating at low speed, with the aid of a brush driven with a translatory motion up to 2 mm. from the edge. After drying, these members were baked at 1l00 (2., this representing an adequate temperature for having minimum resistivity of the equiponderal mixture. The thickness of the semiconducting layer deposited was 0.2 mm. After baking, these rods had values of between 400 and 800 ohms. After threading with a pitch of 1.5 mm., values of 21 kilo-ohms have been obtained :1%. Experiments have shown that in 200 hours at C., the variation of the resistance is of the order of 1%. The dissipation constant is of the order of 20 mw./ C., the same as if the rod were conductive in the mass.

It is obvious that for mixtures to be baked in air and at high temperature (1300 C.), the ceramic material must be chosen accordingly. Account must also be taken of the chemical composition of the latter, because the diffusion of impurities of the substrate into the layer can inhibit reactions giving the semiconductor effect.

It will be apparent that various changes and modifications may be made by those skilled in the art without departing from the essential concept of the invention and which are therefore to be considered as within the scope of the appended claim.

I claim:

1. Method of manufacturing a layer thermistor which permits its ohmic value to be determined with a high degree of precision, which method comprises the steps of coating a refractory substrate with a thin semi-conductive layer having a high temperature coeflicient, baking said layer, measuring the ohmic value of said layer, and adjusting said ohmic value to a predetermined value by cutting a thread into said layer while maintaining the temperature of said layer substantially constant.

References Cited UNITED STATES PATENTS 2,330,782 9/ 1943 Morelock 29620 3,078,550 2/ 1963 Rakowski 29620X 3,248,682 4/1966 Curtis 338300 3,292,130 12/ 1966 Roques 29620X (Other references on following page) UNITED 3 STATES PATENTS Ardouin et a1. 29620X Sapoff et a1. 2962OX Robinson 296-20 Short 338-22X Mandonas et a1. 29-620X Planer et a1. 29620 Grattidge et a1. 29620 4 3,266,661 8/1966 Dates 29620 3,359,632 12/1967 Froemel et a1 2962O JOHN F. CAMPBELL, Primary Examiner 5 R. J. SHORE, Assistant Examiner US. Cl. X.R. 2962() 

